Epoxy resins are widely used in many industrial applications. They are known for their excellent chemical and thermal resistance, good electrical and mechanical properties and for their adhesion to a wide variety of substrates. However, polymers derived from epoxy resins commonly have higher coefficients of thermal expansion (CTE's) than the substrates on which they are placed. Consequently, when the formulations are used as bonding or encapsulation agents, the mismatch between the coefficients of thermal expansion of the polymer and the substrate results in stress, which may cause cracking and loss of adhesion between the epoxide and the substrates. When the substrate is an electronic device, circuit failure results. To circumvent this problem, epoxy resins are often filled with inorganic filler. Typical fillers include particulate silica, ground quartz, alumina and aluminum nitride. However, as the filler loading increases, the viscosity of the formulation increases correspondingly. This phenomenon presents severe restrictions on the potential use of resin formulations having high filler ratios since their high viscosities dramatically increase problems during applying and processing them.
If it were possible to avoid the high viscosities associated with high filler loadings, many additional applications would be immediately affected. High thermal and electrical conductivity, high hardness, high tensile strength and modulus, low shrinkage, and high density, which result from high filler loadings, would also render such materials attractive for four specific applications: die-attach adhesives, polymer bumps, underfill encapsulants, and glob-top encapsulants.
Polymer die-attach adhesives are used to bond a chip or die to a carrier or a circuit board. The die-attach adhesive provides mechanical, electrical and thermal contact between the die and the substrate. The substrate could be a leadframe, a package case, a single or multilayer ceramic, or an organic composite. Die-attach adhesives are sought to replace the expensive gold preform approach previously employed for plastic package applications. Known die-attach adhesives commonly consist of a conductive metal, usually flakes of silver or gold, together with a curable resin, commonly an epoxy or cyanate ester resin. These materials are applied as highly viscous pastes and cured in an oven.
Similarly, a polymer bump is a means of replacing conventional lead-tin solder used to attach dies to the chip carrier or lead frame. Usually, the solder is in the form of small balls placed at specific interconnects around the die. The die is inverted, placed on the substrate and heat is applied to melt the solder, thereby providing both electrical and mechanical connection to the substrate. Polymer-based replacements for solder balls are called "polymer bumps." On heating in an oven, the polymer bump must flow to wet the pads and cure, providing the desired adhesive, electrical and mechanical functions.
Underfill encapsulation is a technique used to reinforce conventional solder bumps connecting the dies to the substrate. A liquid encapsulant comprising a polymer is dispensed along the perimeter of the dies and drawn by capillary action along the surface of the solder bump connection to the substrate of the assembled package. Upon oven curing the encapsulant solidifies and reinforces the solder joints.
Glob-top encapsulation involves dispensing a liquid polymeric material atop a die or chip positioned in a packaging substrate. The polymer is subsequently solidified by thermal curing which provides a protective coating on the die. Generally, a coating between 0.15 and 3.75 mm is applied to the die, depending on the packaging application.
To succeed in these applications, a candidate material must meet the following requirements:
1. give full cure in 60 seconds or less at 200.degree. C. or below for adhesive applications, and in 60 minutes or less at 160.degree. C. or below for encapsulation applications; PA1 2. possess a pot life greater than 24 hours at 25.degree. C.; PA1 3. have a weight loss on cure of less than 2%; PA1 4. have a viscosity suitable for automated dispensing; PA1 5. exhibit no filler settling on storage at 25.degree. C. or at subzero temperatures; PA1 6. have minimal resin bleed (i.e. bleed should be less than 0.125 mm on a variety of substrates); PA1 7. possess low to moderate die and/or substrate warpage after cure; PA1 8. have a low moisture absorption (less than 0.5%) at room temperature (25.degree. C.) or at elevated temperatures (.gtoreq.85.degree. C.); and PA1 9. have excellent adhesion to various inorganic, organic, or metal substrates including solders, solder-masks, and fluxes. PA1 (a) from about 10 to 95 parts by weight of a cycloaliphatic epoxy functional siloxane selected from the group comprising ##STR1## (b) from about 5 to 90 parts by weight of a non-silicon-containing di-, tri-, or polyepoxy resin or mixture of such resins; PA1 (c) from about 0.1 to 3 parts by weight of an iodonium salt of formula ##STR2## wherein M is selected from the group comprising boron, phosphorus, and antimony; X is halogen; n is 4 or 6; and R is selected from the group comprising hydrogen, C.sub.1 to C.sub.20 alkyl, C.sub.1 to C.sub.20 alkoxyl, C.sub.1 to C.sub.20 hydroxyalkoxyl, halogen, and nitro; and PA1 (d) from zero to about 3 parts by weight of a copper compound selected from the group comprising copper stearate, copper naphthenate, copper acetate, copper acetylacetonate, and copper 1,3-pentadienoate.
Considerable effort has been expended by the electronics industry to produce a material that meets the above requirements for adhesive, polymer bump, and/or encapsulant applications. Epoxy resins, typically filled with 60-80% of an electrically conductive filler such as particulate silver or gold, have been proposed for die-attach adhesives. Likewise, epoxy resins, typically filled with 60-80% of an electrically nonconductive particulate filler such as silica, have been proposed for underfill and glob-top encapsulants. These epoxy-based materials appear to be suitable as adhesives or encapsulants because of their chemical resistance, electrical properties, thermal stability, and processability. However, from a polymeric materials standpoint, many of the aforementioned properties are interdependent, and one property of the system cannot be enhanced without altering another. In addition, epoxies tend to exhibit a poor shelf life, high modulus, and slow cure under normal curing conditions. Therefore, due to the aforementioned problems and disadvantages, no epoxy-based materials are currently available that meet all the above-listed requirements for die-attach adhesives, polymer bumps, underfill, and/or glob-top encapsulants.